diff --git a/src/exdf/write/sd_writer.py b/src/exdf/write/sd_writer.py
index 0ea9e4595feede6cbf81062c141830b5dd813430..cdeae23d4ea6831272b3ca5f1c2f824f2d4215c9 100644
--- a/src/exdf/write/sd_writer.py
+++ b/src/exdf/write/sd_writer.py
@@ -430,6 +430,7 @@ def get_key_attributes(sd):
 
     return source_attrs
 
+
 def iter_index_group_keys(keys, index_group):
     for key in keys:
         if key[:key.index('.')] == index_group:
@@ -446,11 +447,11 @@ def mask_index(g, counts, masks_by_train):
     # Modify INDEX entry if necessary.
     if full_mask.sum() != num_entries:
         g.create_dataset(
-            f'original/first', data=get_pulse_offsets(counts))
+            'original/first', data=get_pulse_offsets(counts))
         g.create_dataset(
-            f'original/count', data=counts)
+            'original/count', data=counts)
         g.create_dataset(
-            f'original/position',
+            'original/position',
             data=np.concatenate([np.flatnonzero(mask)
                                  for mask in masks_by_train]))